Hassan, All cisco products support M13 framing.
M23 is multiplexor framing from 2nd level (8 mbps) to 3rd level (35 or 45 mbps). That thing doesn't never got application in the reality of telecomms.
So you're with nm-1T3/E3 on the 3800 for high-speed FR service.
Hope this helps, please rate post if it does!